1. Field of the Invention
The invention relates to a method and contact slide unit for a switching unit, having a contact slide with a movable switching piece guided therein.
2. Description of the Related Art
Switching units, in particular circuit breakers, are used inter alia for safe disconnection in the event of a short circuit and, thus, protect consumers and installations. Furthermore, electrical or mechanical switching units are suitable for the operationally dependent, manual switching of consumers and for the safe isolation of an installation from the electrical grid in the event of maintenance work or changes to the installation. Electrical switching units are often operated electromagnetically.
In other words, such switching units are electrical switching devices that are high quality in technical terms with integrated protection for motors, lines, transformers and generators. Switching units are used at service facilities with a low switching frequency. In addition to short-circuit protection, such switching units are also suitable for overload protection.
In the event of a short circuit, an electrical switching unit disconnects an electrical installation safely. Thus, this electrical switching unit provides safety protection from overload. Any conductor through which current is flowing is heated to a greater or lesser extent. In this case, the heating is dependent on the ratio of the current intensity to the conductor cross section, known as the current density. The current density should not become too great because otherwise the conductor insulation can be scorched by excessive heating and possibly a fire can be triggered.
Circuit breakers have two tripping mechanisms that act independently of one another for overload and short-circuit protection. Both releases are connected in series. In the event of a short circuit, an electromagnetic release that acts virtually without any delay in time performs the protection function. In the event of a short circuit, the electromagnetic release unlatches a switching mechanism of the circuit breaker without any delay. A switching armature isolates the switching piece before the short-circuit current can reach its maximum value.
In circuit breakers having a high switching capacity (ICU up to 100 kA at a rated current of IN=100 A), high short circuits occur, with the result that in turn large magnetic forces arise between the movable switching piece and the fixed switching piece. These are, for one part, current-loop forces between the fixed switching piece and the movable switching piece and, for another part, the much greater current constriction forces between the silver contacts. These two forces have the effect that, in the event of a short circuit, the movable switching piece is suddenly thrown counter to the resulting spring force thereof and strikes the impact domes in the lower part.
Furthermore, the abovementioned current constriction forces between the silver contacts do not act centrally in the middle of the contacts but decentrally further toward the periphery of the contacts. In addition to the opening force in the direction of the impact domes in the lower part, the movable switching piece therefore experiences a torque that can result in rotation of the movable switching piece about the longitudinal axis. In addition, the striking of the impact domes, can also have the effect, given an unfavorable position of the movable switching piece, that an undesired torque is again initiated.
Under extreme load, it is occasionally possible for the movable switching piece to exploit its design-determined degrees of freedom and, as a result of the initiated torques, for it to either become wedged in the contact slide or rotate so far that regular contact between the contacts of the movable switching piece and the fixed switching piece is no longer possible. In any case, reliable operation of the circuit breaker is no longer ensured.
Known contact slides of these slide units frequently have two guide systems, i.e., an internal guide system and an external guide system. The external guide system is used when the switching operation, i.e., the switch-on or switch-off operation, occurs via a switching mechanism of the switching unit. In this case, no bridge rotator occurs.
The internal guide system is used in the event of a short circuit when the switching operation is performed via a switching armature, frequently a plunger, of the switching unit. In other words, in the event of disconnection on account of a short circuit, the movable switching piece extends ahead of the contact slide along the internal guide system, rebounds at the impact faces in what is referred to as the lower part of the switching unit and flies back along the internal guide system again. In this case, it flies in the opposite direction to the switching armature or the plunger of the switching unit. Here, it is possible for the movable switching piece and the plunger to meet one another outside their center lines, and this can lead to rotation of the movable switching piece about its longitudinal axis.
If the movable switching piece remains in the rotated state, when the switching unit is next switched on, the contacts, in particular silver contacts of the movable switching piece and the fixed contacts of the switching unit, no longer meet one another, with the result that the failure phenomena occurs. In other words, a switching piece that remains in a rotated position is disadvantageous because the switching unit is then no longer usable. A non-functioning switching piece and a non-functioning switching unit are disadvantageous for the electrical consumers and the installation in which the switching unit is installed.